1. Field of the Invention
The invention relates generally to non-volatile memory devices, and particularly to an improved method and apparatus for testing non-volatile memory devices.
2. Description of the Related Art
Non-volatile memory refers generally to all forms of solid state memory that does not need to have its memory contents periodically refreshed. Common examples include all forms of read-only memory (ROM) such as programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), and flash memory.
Flash memory is a variation of EEPROM that is erased and rewritten in blocks instead of at the byte level. Flash memory is increasingly being used in digital cellular telephones, digital cameras, LAN switches, PC cards for laptop computers, digital set-top boxes, embedded controllers, and other devices.
The data in flash memory is stored in memory elements addressed by a row line and column line. Each memory element contains a charge storage structure, such as a floating gate. The amount of charge in the charge storage structure determines the logical state of the memory element. If the amount of charge stored is greater than a pre-defined threshold, the logical value stored in the memory element is a logical value of 0; otherwise, the memory element contains a logical value of 1.
In order to deliver zero defect products, manufactures utilize Vt (threshold voltage) distributions to identify bits that don't meet threshold voltage specifications (“maverick bits”) as well as the healthiness of the manufacturing process. The Vt distributions are obtained by Automated Tester Equipment (ATE) by applying a specific voltage to the flash module and counting the number of cells with a value of 0 or 1. The process is repeated for a range of voltages to determine the Vt distribution for the module. For each cell during each cycle, the ATE must read and accumulate the cell values. The data transfer from the flash module to the ATE is very slow and acts as a bottleneck during the module verification process.
Moreover, the creation of high density and high performance flash modules allowed by improvements in design and manufacturing has increased module sizes such that they require an even greater amount of testing to be performed on the module and data exchange between the module and the ATE. Using current methods of testing, a 2 megabyte flash module will take between 10 to 15 minutes to collect a Vt distribution. As module densities increase, module test times will also increase. Therefore, there exists a need for a new and improved method and system for threshold voltage testing to more effectively accommodate high density memory modules.
In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration of specific exemplary embodiments in which the invention may be practiced.